1. Field of the Invention
This invention relates to a method of producing a homogeneous silicon nitride sintered body. The method causes no change in the quality, color, or the like, of the silicon nitride sintered body and the body has high bending strength and the like.
2. Related Art Statement
Silicon nitride (Si.sub.3 N.sub.4) sintered bodies are excellent in thermal resistance, thermal shock resistance, corrosion resistance and high temperature strength, and are used as high temperature structural materials, wear resistant materials or the like, for gas turbine engines, diesel engines and so on. However, in silicon nitride sintered bodies there is apt to be a great difference in sintering properties between the interior and surface of the sintered body due to decomposition of silicon nitride and evaporation of sintering aid produced in the firing. Therefore, in order to suppress the unhomogenization of the sintering properties, the firing is carried out in a sagger.
As the sagger for firing silicon nitride sintered bodies, there have hitherto been known a carbon sagger, a carbon sagger covered with a chemically deposited silicon carbide (Japanese Patent laid open No. 60-210,575), a carbon sagger covered at its inner face with silicon carbide formed by a reaction with carbon in the sagger (Japanese Patent Application Publication No. 61-3,304), a sagger composed of an inner silicon nitride sagger and an outer refactory sagger (Japanese Patent Application Publication No. 49-40,123), and a sagger mainly composed of graphite and silicon carbide (Japanese Patent Application Publication No. 33-4,075).
In the carbon sagger, however, the evaporation is large on the fired surface of silicon nitride sintered body. Further, SiC is liable to be produced on the surface, so that the strength at the fired surface becomes undesirably small.
In the carbon sagger covered with chemically deposited silicon carbide, peeling often occurs between the carbon layer and silicon carbide layer, and thus the durability of the sagger is small.
In the sagger covered at its inner face with silicon carbide formed by a reaction with carbon in the sagger, the silicon carbide surface layer is porous, so that the effect of covering the carbon with a silicon carbide layer is substantially small. Further, the evaporation on the fired surface of silicon nitride is large, as in the case of the carbon sagger, and thus the strength at the fired surface is low. Moreover, the change of quality and color such as stripes, spots and the like are apt to be produced in the surface layer of the sintered body.
In the sagger composed of an inner silicon nitride sagger and outer refractory sagger, softening is liable to be caused in the firing, so that the firing of shaped bodies can not be carried out in a large weight or large amount. Furthermore, the surface is apt to be decomposed to degrade the durability. Moreover, the quality and color changes such as stripe, spots and the like are apt to form in the surface layer of the sintered body.
In the sagger mainly composed of graphite and silicon carbide, the whole of the sagger is low in density and highly porous, so that the evaporation on the fired surface silicon nitride is large and the strength at the as-fired surface is small. Further, the quality and color changes such as stripe, spot and the like are apt to be caused in the surface layer of the sintered body.
In these conventional saggers, there are problems on the tightness, durability and the like. When using these saggers, the resulting silicon nitride sintered bodies do not yet have sufficient properties, because the quality and color changes are caused in the surface layer.